1. Technical Field
The present invention relates to a fixing device to perform fixation of unfixed images carried on a recording medium by passing the medium though a fixing nip, and further relates to an image forming apparatus incorporating such a fixing device, such as a copier, a printer, a facsimile machine, a plotter, or a multi-function apparatus having one or more of these capabilities.
2. Related Art
Demand for faster, more energy-efficient image forming apparatuses such as printers, copiers, and facsimile machines is acute.
In these types of image forming apparatuses, a toner image is formed by image forming processes such as electrophotographic recording, electrostatic recording, magnetic recording, and the like. The toner image is then transferred onto a sheet such as a sheet of paper, a printed sheet, a photo-sensitive paper, or a dielectric-coated paper (hereinafter, simply also “recording medium”), via an indirect transfer method or a direct transfer method.
As a fixing device to fix an unfixed toner image, fixing devices employing a contact heating method are widely used. These methods include a heat-roller method, a film or belt heating method, and an electro-magnetic induction heating method.
Of these methods, a thin fixing member having a low thermal capacity is optimal for reducing both warm-up time of the fixing device and power consumption. Herein, “warm-up time” means a length of time that the fixing device takes to heat up from room temperature when power is turned on up to a predetermined temperature, i.e., a reload temperature, capable of performing printing. JP-2004-286922-A discloses a fixing device employing a belt as an example.
Regarding the fixing devices employing a belt, there is demand for a further reduction of both warm-up time and of first-print time. Herein, “first-print time” means the time it takes, after receipt of a print request, to print out a sheet. However, the faster the printing speed, the number of prints per unit time increases and a required heat quantity increases drastically. In particular, there is not enough heat at the start of continuous printing, that is, a temperature drop is a problem.
To solve the above problem, JP-2005-092080-A discloses a fixing device employing an endless belt, in which the entire belt is heated. With this structure, the first-print time from a standby time can be shortened and the heat shortage in high-speed printing can be prevented. As a result, if mounted on a high-speed image forming apparatus, such a fixing device provides optimal image fixability.
Most fixing devices employing such a fixing belt are configured to rotate driven by a pressure roller disposed opposite the fixing belt. More specifically, the fixing belt is pressed by the rotating pressure roller and the fixing belt rotates together with the pressure roller because the force of friction exceeds the torque required to rotate the fixing belt. Thus, the fixing belt rotates driven by the pressure roller.
In such a fixing device, if the force of friction that the fixing belt receives from the pressure roller is below the torque required to rotate the fixing belt, the fixing belt slips at the fixing nip. As a result, the fixing belt cannot rotate anymore and stops. In particular, if the coefficient of friction of the surface of the recording medium which passes through the fixing nip is small, the fixing belt receiving friction from the pressure roller when the recording medium is sandwiched between the pressure roller and the fixing belt is reduced compared to a state in which no recording medium is supplied.
JP-2005-092080-A discloses a fixing device in which the fixing belt is configured to slide on the biasing member that presses the fixing belt against the pressure roller, so that the fixing belt receives a greater sliding friction from the biasing member. As a result, a rotary torque of the fixing belt becomes large, which causes slipping.
As a method to prevent the above problem, one approach involves increasing the force of friction exerted by the pressure roller. JP-2012-103402-A discloses a structure that disposes a grip portion at both ends of the pressure roller and the fixing belt. The grip portion allows the pressure roller and the fixing belt to contact each other without intercession of the recording medium even while it is being conveyed, and the force of friction exerted on an outer circumferential surface of the grip portion is maintained at a certain level or more. With this structure, the slipping of the fixing belt can be prevented regardless of the coefficient of friction of the recording medium.
However, although such a slipping prevention method is applied to the fixing device, the force of friction that the fixing belt receives from the pressure roller is reducing with time in the continued use of the fixing device. If the force of friction received from the pressure roller is reduced, the slipping of the fixing belt tends to occur. In particular, foreign particles such as powdery dust or oil attached to the surface of the fixing belt or the pressure roller are a problem. In this case, the sheet serves to clean the fixing belt and the pressure roller and removes the foreign particles therefrom in the area through which the recording medium passes.
On the other hand, in an area of the fixing belt and/or the pressure roller on or over which the recording medium is not conveyed, the foreign particles attached to the fixing belt or the pressure roller are not removed and so gradually accumulate. Such foreign particles reduce the coefficient of friction of the fixing belt or the pressure roller at the fixing nip and reduce the force of friction that the fixing belt receives from the pressure roller. As a result, slipping tends to occur when the recording medium passes though the fixing nip.
To cope with this problem, JP-2011-174955-A discloses a structure to dispose a shutter member on the surface of the pressure roller. The shutter member prevents an inflow of the foreign particles from the sheet-passing area to the non-sheet-passing area. As a result, the force of friction that the fixing belt receives from the pressure roller is prevented from being reduced even with continued operation of the fixing device.
However, flows of the foreign particles into the area through which the recording medium does not pass are not limited from the area through which the recording medium passes. The structure disclosed in t JP-2011-174955-A does not prevent foreign particles such as powdery dust or scattered toner floating inside the image forming apparatus from attaching to the fixing belt or the pressure roller in the area through which the recording medium does not pass. If the foreign particles attach to the non-sheet-passing area, the force of friction that the fixing belt receives from the pressure roller reduces. Once the force of friction is reduced, even the shutter member does not contribute to recovery of the force of friction.